1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member comprising a specific resin to be electrostatically charged directly by a charging means. This invention also relates to a process cartridge and an electrophotographic apparatus, each having such an electrophotographic photosensitive member.
2. Related Background Art
When images are formed on an electrophotographic photosensitive member in electrophotography basically comprising the steps of charging, exposure, developing, transfer, fixing and cleaning, usually the charging has been carried out by so-called corona charging, where the photosensitive member is electrostatically charged with corona discharge by applying a high voltage (DC 5 to 8 kV) to a metal wire. This method, however, involves problems due to by-products of corona discharge, such as ozone and NO.sub.x. They adhere to the surface of the photosensitive member or change the properties of the photosensitive member to cause smeared images and blurred images, and soil the wire so as to cause blank areas and black lines in images. In particular, photosensitive members, the photosensitive layer of which contains an organic photoconductive material, have a lower chemical stability than those making use of other inorganic photoconductive material such as selenium or amorphous silicon. Therefore, upon the exposure to the by-products of corona discharge, chemical reactions (mainly oxidation) are liable to occur, to thereby to cause deterioration of the photosensitive member.
Another problem of corona charging is its poor electric power efficiency for a charging means, since only 5 to 30% of electric current is directed to the photosensitive member, and the rest flows into a shielding plate.
As measures for solving such problems, Japanese Patent Application Laid-open Nos. 57-178267, 56-104351, 58-40566, 58-139156, 58-150975 and so forth disclose so-called direct discharging, in which a voltage is applied to a charging member in contact with the surface of a photosensitive member to electrostatically charge the photosensitive member not using a corona discharger. Particularly, this is a method in which a charging member such as a conductive elastic roller is brought into contact with the surface of a photosensitive member and a DC voltage of about 1 to 2 kV is externally applied to this charging member to electrostatically charge the surface of the photosensitive member to have a given potential.
Such direct charging, however, has a problem that the charging often becomes uneven. The uneven charging means the entire surface of the photosensitive member is not evenly charged, with defectively charged areas such as spots, or longitutional lines of about 2 to 200 mm long and about at least 0.5 mm wide. This results in image defects such as white dots or white lines (phenomena in which white spots or lines appear in black solid images) in normal development, and black dots or black lines (phenomena in which black spots or lines appear in white solid images) in reversal development.
Accordingly, in order to improve the uniformity of charging, a method is proposed in which a pulsating current voltage comprised of a direct current voltage (V.sub.DC) superimposed with an alternating current voltage (V.sub.AC) is applied to a charging member (Japanese Patent Application Laid-open No. 63-149668). In this case, in order to prevent image defects such as white dots and white lines or black dots and black lines by maintaining the uniformity of charging, the superimposed AC voltage must have a peak-to-peak potential difference (V.sub.P--P) at least twice the DC voltage.
However, when the AC voltage is increased to prevent image defects, the maximum applied voltage of the pulsating current voltage also increases. Thus, discharge insulation breakdown will occur even at places having very slight defects inside the photosensitive member. In particular, in the case of photosensitive members making use of an organic photoconductive material of a low withstand voltage strength, the insulation breakdown is very liable to occur. In such an instance, in the normal development, blank images may appear in the lengthwise direction of contact areas and, in the reversal development, black strips.
Superimpose of V.sub.AC means that a high-frequency current is applied to a charging member which has been brought into contact with an electrophotographic photosensitive member. Hence noise may be generated at the time of charging. Further, since currents of the AC-component flow into the electrophotographic photosensitive member in excess, electrical deterioration such as an increase of residual potential often occurs, or the surface of the photosensitive member is often damaged in part where the toner adheres to cause so-called melt-adhesion.
Under such circumstances, a system to apply only V.sub.DC without superimposing V.sub.AC has been revaluated. There, however, still remains the problem of the nonuniformity of charging, as previously discussed.
Recently, as the image quality is improved, there has become a problem of so-called transfer memory, a phenomenon in which the capacitance inside the photosensitive member changes as a result of transfer charging, causing a difference in sensitivity between the area subjected to transfer charging and the area not subjected to transfer charging.